Optical signals suffer degradation between the transmitter and receiver from such factors as noise, inter-symbol interference, fiber dispersion, non-linearity of the elements and transmission medium. In addition, in amplified wavelength division multiplexed (WDM) systems, the transmission characteristics vary from one channel to another due to the non-flat gain and noise profile of erbium-doped fiber amplifiers (EDFAs).
Distortion is defined as any inaccurate replication of a signal transmitted over a communication link, and could originate in any network element (NE) along the link. It can be measured by assessing the difference between the wave shape of the original signal and that of the signal at the network element of interest, after it has traversed the transmission link.
In the last decade, transmission rates of data signals have increased progressively, which has led to more complex and less tolerant transmission systems. For transmission at high rates, such as 40 or 80 Gb/s, the distortion of the optical link is a critical parameter. With various types of dispersion shifted fiber, dispersion compensating fiber and dispersion compensating elements that make up a given link, determining the cause of a distortion in the received signal is no longer a simple operation, especially in optical transmission systems with in-line optical amplifiers. System performance degradation caused by noise and optical path distortions are also usually difficult to separate, making the performance evaluation complicated.
In the evaluation of the characteristics of an optical fiber communication system, the bit error ratio (BER) has usually been used as a parameter for performance evaluation. BER is defined as the ratio between the number of erroneously received bits to the total number of bits received over a period of time (for example a second). A number of codes have been provided in the signal at transmitter for error detection, the basic idea being to add redundant bits to the input data stream over a known number of bits. The BER calculated by the receiver includes information on all impairments suffered by the signal between the transmitter and receiver, i.e. both noise and distortion together.
Performance of an optical system is also defined by a parameter called Q. The Q value (or Q-factor) indicates the ‘useful signal’-to-noise ratio of the electric signal regenerated by the optical receiver, and is defined as follows:       [          [              Q        =                                            μ              1                        -                          μ              0                                                          σ              1                        +                          σ              2                                          ]        ]        Q    =                            μ          1                -                  μ          0                                      σ          1                +                  σ          0                    where μ1 is the mean value of the ‘1’s, μ0 is the mean value of the ‘0’s, σ1 is the standard deviation of the level of ‘1’s, and σ0 is the standard deviation of the level of ‘0’s. These parameters can be understood from looking at the so-called eye diagram, which represents the received signal, time-shifted by integer multiples of the bit period, and overlaid. The eye diagram can be produced on an oscilloscope by applying a baseband signal to the vertical input of the oscilloscope and triggering the instrument time base at the symbol rate. For a binary signal, such an eye diagram has a single ‘eye’, which is open or closed to an extent determined by the signal degradation. An open pattern is desired, as this provides the greatest distance between signals representing a 1 and those representing a 0. Changes in the eye size indicate inter-symbol interference, amplitude irregularities, or timing problems, such as jitter, depending on the signal that is measured.
An eye diagram is shown in FIG. 1, with representation of the μ and σ values. The point 8 at which the probability density curves cross represents the decision threshold position to give the lowest error ratio. The value of Q can be used to derive the bit error ratio (BET) using the equation:   BER  =            1      2        ⁢          erfc      ⁡              (                  Q                      2                          )              ⁢                  ⁢    where    ⁢                  ⁢    erfc    ⁢                  ⁢    is    ⁢                  ⁢    the    ⁢                  ⁢    complimentary    ⁢                  ⁢    error    ⁢                  ⁢          function      .      
Approximately, a Q value of 6 represents one error in 109, and a Q value of around 7 represents one error in 1012. Optical systems have very low BERs under nominal conditions of operation, and therefrom measurement of BER under normal operating conditions is extremely time consuming. The measurement of the Q value is much quicker, but can still be too slow for some applications. In order to enable the Q value to be used more practically for error prediction purposes, it has been proposed to sweep the decision threshold of the receiver through all voltages from the voltage level corresponding to a zero to the voltage level corresponding to a one. For example, when the decision threshold is near the zero voltage level, there will be no errors in interpreting a “1”, even if there is significant distortion. There will, however be a greatly increased error ratio in interpreting the zeros. The BER is measured for each decision threshold voltage, and by mapping the BER values using an appropriate function, a straight line extrapolation can be used to obtain the Q value, which then can be used to derive the BER for the actual decision threshold voltage, even though no errors may have been recorded for that decision threshold.
FIG. 2 shows a plot of the offset of the decision threshold voltage from the mid point (V) against a function (F) of the measured BER values. This function is:√{square root over (2)}·srfc−1(BER×4)The apex 10 of two straight line fits 12,14 provides the optimum Q value (and the decision threshold required to achieve this value).
Although this approach avoids the need to take BER measurements where there is a very low incidence of errors (there are no measured values near the apex 10), accurate evaluation of the Q value does nevertheless require BER measurements to be taken on the low error ratio part of the curve. Typically, it may take at least 10 seconds to obtain one Q measurement (depending upon the desired accuracy). The evaluation of errors is required on a channel by channel basis, so that for a WDM system supporting 100 channels, this would require a cycle of at least 1000 seconds to measure the Q value of all channels. Measuring the Q value for each channel in turn results in an unacceptable delay between successive measurements on an individual channel.
U.S. Pat. No. 5,585,954 discloses an apparatus for measuring Q values, in which the time is reduced by measuring the BER for all decision threshold simultaneously. This of course requires a number of decision circuits which can simultaneously measure the BER when applying different decision thresholds. This complicates and increases the cost of the measurement circuitry.